The majority of rice (Oryza sativa, L.) crops that are grown and consumed throughout the world are white-grained varieties, however rice grain may also be brown, red and/or purple. Rice plants producing red grain are ubiquitous among wild rice plants and in some regions of the world rice cultivars producing red grains are preferred over white grains for their taste, texture and ceremonial or medicinal value. Consumer interest in red or purple rice grains represents a growing specialty market in the United States. However, the ubiquitous presence of red rice plants as a weed in farmers' white rice plant fields is the most economically important pest and grain quality problem faced by United States rice growers today, leading to losses of as much as $50 million per year (Gealy et al. (2002) Weed Science 50:333-339). In fact, in the United States the United States Secretary of Agriculture classifies red rice as a “Noxious Weed” such that importation and red rice plant cultivation is highly regulated.
Although naturally occurring relatives of Oryza sativa (Asian cultivated rice) are not native to the United States, weedy rice plants producing red grain are a constant problem in the rice growing areas of the United States. These weedy red rice plants were ubiquitous contaminants of the original white rice plants imported into the United States where efforts to remove them completely from the white rice growing areas have been ineffective, in part due to red rice plants out-crossing with cultivated, white-grained varieties and due to growth of red rice plants in uncultivated areas near cultivated white rice fields. These weedy rice plants exhibit increased dormancy and shattering allowing red rice plants to persist in white plant rice fields despite vigorous attempts to remove them. Efforts to manage the problem genetically have been fraught with difficulties, in part because both red and white-grained varieties appear to be the same color until the husk is removed. Thus millers must dehusk and sample each load of rice in order to determine the degree of red rice contamination. Further, breeding efforts are difficult because rice plants in general may display an unstable color phenotype, making it difficult to genetically track the character in breeding populations. While the red color can be removed by polishing in order to provide white grains, red rice must be polished longer than white rice to remove all traces of color, with the increased polishing resulting in more broken grains and a decrease in market value of red rice. Thus causing a decrease in market value of white-grained rice crops with substantial red rice contamination.
Recent efforts to control red rice include transgenic herbicide resistance systems such as the Clearfield-Newpath™ system wherein transgenic white rice plants “Clearfield™” expressing resistance to Newpath™ herbicide allow the removal of red rice plants susceptible to Newpath™ herbicide. However there is recent evidence that the white rice plants are cross-pollinating with red rice plants thus transferring the herbicide resistance to the weedy red rice plants.
Thus, there is a need to identify molecular components and methods for the production of red pigmentation in rice. These novel components will offer new methods for identifying and controlling “red rice” plants for seed and grain certification analysis and further for breeding strategies aimed at minimizing the extent of the red rice problem in the United States and internationally.